a(n)=2(4^n-n*2^n)-1.

Terms

a(0) =1a(1) =3a(2) =15a(3) =79a(4) =383a(5) =1727a(6) =7423a(7) =30975a(8) =126975a(9) =515071a(10) =2076671a(11) =8343551a(12) =33456127a(13) =134004735a(14) =536412159a(15) =2146500607a(16) =8587837439a(17) =34355281919a(18) =137429516287a(19) =549735890943